With the increasing concerns for the safety of the workplace and the quality of air, those industries which discharge gases containing particulate byproducts of their processes or which tend to create dust in their workplaces commonly use filtration of the gas or air as one method of solving the problem. These industries include, by way of example, the electric and gas utilities, refineries, asphalt and cement producers, foundries, and carbon black producers.
Since the environmental regulations are becoming significantly more strict, the foregoing industries have had to adapt highly efficient particulate removal methods and devices and give considerable attention to the control of very fine particles, trace element emissions, and plume opacity. To meet these stricter environmental regulations many of these industries have begun using a greater number of glass fabrics in their filtration systems. Glass fabrics offer many advantages, being inherently inert, but improvement is desired, particularly, in flex life after exposure to acids or alkalies. For example, as gases pass through the filtration fabric, dust particles are collected in the form of a "cake" which then becomes part of the filter medium. The collected dust or cake must be periodically removed from the filter to prevent significant loss of gas flow and this can be done during a cleaning cycle by reverse air, shake-deflate, and pulse jet methods. These cleaning cycles which require repeated flexing cause fabric fatigue so that an improved flex life is of prime importance.
One well-known system of gas filtration employs a plurality of filter bags which may be made from glass fiber. These bags are suspended, usually open end down, in closed compartments which form a structure known in the trade as a "bag house." As an example of one method of filtration and the arrangement of a bag house fabric filtration system, reference is made to U.S. Pat. No. 4,523,931, which issued to Richard L. Miller et al. An illustration of a method of cleaning a filter by pulsed air is disclosed in U.S. Pat. No. 4,331,459 which issued to George A. Copley on May 25, 1982. Another type of filter system, one for agglomerating and removing carbon black, is set forth in U.S. Pat. No. 4,579,567, which issued to Paul H. Johnson on Apr. 1, 1986. For a specific flexible filter bag design, reference is made to U.S. Pat. No. 4,877,526 which issued on Oct. 31, 1989 to Todd W. Johnson et al; and, for still another bag house and hanger arrangement reference is made to U.S. Pat. No. 3,710,559 which issued on Jan. 16, 1973 to Dale K. Harris. In each of the foregoing patents, methods and apparatuses are described for using filters which require that the filter fabrics meet many exacting requirements including the need to be flexible and withstand the repeated cycles of flexing caused by back flow of cleaning air and other pressure changes. Accordingly, it is a general object of the present invention to provide gas filtration fabrics which will withstand a very high number of flexing cycles and the stress of reversed air flow.
As will be readily recognized about the foregoing mentioned industries and the gases discharged in their processes, the gases are often heated, and, in many instances, contains the products of incomplete combustion as well as complete combustion including the dust particle matter associated with the particular industry and its processes. A specific example is the burning of high sulphur content coal by the electric utilities. Thus, acids and alkalies at high temperatures in the range of as high as 300.degree. F. to 500.degree. F. or more in the discharge gases are to be expected and must be successfully accommodated. As mentioned above, when first placed in service, gases passing through a filter deposit particles therein and as the deposits build up the filter becomes more effective because the particles become part of the filter. This build up or "cake" reaches a point where flow becomes reduced and the pressure drop across the filter becomes unacceptably excessive. Flexing the filter by reversing the air flow is one way of removing the cake. Condensate can collect in the cake thus adding to the removal problem; and, flexing a filter to clean it after exposure to these chemically hostile conditions can result in breakdowns and ruptures of the fiber filaments within the fabric. Eventually, failure of the filter occurs thus requiring replacement of a bag or bags. Unplanned or too frequent filter replacements can be quite costly. Even with bypass systems which can shunt the gas discharge flow from one path to another, it is highly undesirable to have a failure in the bypass path while replacing filters in the primary path. Thus, another general object of the present invention is to provide a filter fabric which will reliably withstand the severe, adverse conditions caused by heated gases containing chemically reactive products.
To improve the strength, flexibility, and all around performance of glass fabrics, it has been a common practice to impregnate the fiber with one or more polymeric materials. In U.S. Pat. No. 4,450,197 to Hager et al, which issued May 22, 1984, a method of impregnating a bundle of glass fibers with a polytetrafluoroethylene resin and a film-forming acrylic resin is disclosed which renders the fibers suitable for filtering fabrics for filtering flue gases. Other disclosures of glass fiber coated or loaded with fluoropolymers are U.S. Pat. No. 4,645,709; U.S. Pat. No. 4,323,603; and U.S. Pat. No. 3,790,403. It is desirable to have a coating or finish on the woven fibers that will reduce fiber-to-fiber abrasion. Coatings of fluorocarbon polymers are attractive because of the reduction in surface friction they offer. However, this same attribute makes bonding to a substrate difficult. Accordingly, another object of the invention is to provide an improved method of bonding fluorocarbons to a glass fiber substrate.
A specific object of the present invention is to provide a filtration fabric which has an extended life and reduces the number of times required to change filter bags.
A further object of the present invention is to provide a filtration fabric with improved resistance to acid and alkali failures.
A still further object is to provide fabric which has extended flex life and is woven from glass fibers and/or other fibers
These and other objects are accomplished by the present invention which is described more fully below.